1. Field of Invention
The techniques described herein relate generally to voltage regulators. Some embodiments relate to a voltage regulator having a fast transient response and operable over a range of load capacitances. The voltage regulator can operate over a range of load capacitances without an external capacitance to stabilize the regulator. Some embodiments relate to a low-dropout (LDO) voltage regulator operating without a stabilizing external capacitor.
2. Discussion of Related Art
Electronic circuits are often designed to operate using particular supply voltages. A circuit may function improperly when the supply voltage is not at the proper value.
Voltage regulators are used to provide constant supply voltages to circuits despite variations in a power source and/or in the circuit elements. The voltage regulator is connected between a power source and the circuit it supplies. The voltage regulator includes components to regulate a voltage output by the voltage regulator and to monitor that output voltage for the purpose of regulation. The regulator is designed to provide a constant output voltage, but the output voltage of the regulator may vary if there is a variation in the input from the power source and/or if the circuit being powered draws more or less current at a given time (e.g., as the load varies). As the output voltage varies, the regulator operates to compensate for the variation to provide a constant voltage output.
One type of a voltage regulator is a linear voltage regulator, an example of which is shown in FIG. 1. The linear voltage regulator 100 of FIG. 1 operates based on an input supply voltage Vin from a voltage source 102 and operates to maintain an output voltage Vout at a constant level based on a reference voltage Vref. The regulator 100 does this using a voltage-controlled current source 104, producing an output current Iout that varies based on variations in Vout. Iout is regulated such that it will yield the desired voltage Vout, at a constant level. The current Iout is also regulated to provide the constant Vout based on a level of the input voltage Vin. The voltage-controlled current source 104 is controlled to ensure that the output current Iout appropriately varies as the resistance 106 (Rload) changes and/or the input voltage Vin changes.
To control the current source 104, the regulator 100 includes a resistor network of resistor 110 and resistor 112 that produces a voltage Vsense indicative of the voltage Vout. As Vout varies due to a varying current Iload drawn by the load circuit on the regulator 100 and/or due to a varying input Vin, the voltage Vsense will also vary. Voltage Vsense is input to an error amplifier 108, implemented using an operational amplifier (“op-amp”). The error amplifier 108 compares the voltage Vsense to the reference voltage Vref and outputs an error voltage Verror indicating a voltage difference between Vsense and Vref. This voltage Verror is then used to control the voltage-controlled current source 104 to output a modified current Iout such that the voltage Vout is maintained substantially constant.
The variations in Vout are known as “transients.” A transient is characterized as fast or slow, depending on how quickly the change occurs or how long the change lasts. The period of time from when Vout first varies from Vref to the time it settles again to Vref—in other words, the time for the regulator 100 to respond to variations in Vout—is known as the “transient response time.” Different types of regulators may have different transient response times. Fast transients may sometimes result in errors in the load circuit if the regulator 100 cannot respond quickly enough to the transient (i.e., if the transient response time of the regulator is slower than the speed of the transient) and the voltage Vout varies too much or too long from the constant level expected by the load circuit.
FIG. 2 shows one type of linear voltage regulator, known as a low dropout (LDO) voltage regulator. The drop-out voltage of a regulator is the minimum voltage drop across the regulator needed to maintain the expected output voltage Vout. A lower drop-out voltage means less energy is consumed by the regulator and thus the regulator has a higher efficiency. An LDO regulator has a low drop-out voltage and can be desirable for many applications that need to conserve energy (e.g., battery-powered devices).
As in the regulator 100 of FIG. 1, the LDO regulator 200 receives an input voltage Vin and provides an output voltage Vout to a load circuit, and includes a resistor network of resistors 110 and 112 providing a voltage Vsense to an error amplifier 108. The voltage-controlled current source of the regulator 200 is implemented using two transistors 202 and 204. The resistor 206 draws a current from the amplifier 108 based on the voltage Verror, and that current is provided at the base of the transistor 202 to control the current flowing from the collector to the emitter of the transistor 202. The current flowing from the collector to the emitter of transistor 202 is a current drawn on the base of transistor 204, which controls the current flowing from the emitter to the collector of transistor 204. The current flowing from the collector of transistor 204 is output as the output current Iout of the regulator 200. The transistors 202 and 204 and the resistor 206 thus act as a voltage-controlled current source that is controlled based on the voltage Verror, as in regulator 100 of FIG. 1.
Some regulators, particularly the LDO regulator, function properly only for certain types of load circuits that have certain characteristics. For example, the regulators will work properly for load circuits that have a resistance within a certain range, have a capacitance within a certain range, and/or draw a current within a certain range. Outside of those ranges, the feedback loop of the regulator that controls the current source will be unstable. When unstable, the regulator cannot properly regulate the output voltage in responses to transients, and thus the voltage output Vout will continue to vary for a long time or indefinitely, causing a large or potentially infinite transient response time. Linear voltage regulators that are used with circuits that change characteristics quickly or to a large degree are particularly susceptible to becoming unstable. If characteristics of a load circuit change quickly as a result of a change in operations in a circuit, then the fast transient may cause the voltage regulator to become unstable and stop working properly. Similarly, a large transient can cause instability in the regulator.
To diminish the risk of this instability occurring and enable the regulators to work accurately with more types of load circuits, regulators (particularly LDO regulators) are used with external capacitances that are coupled to the output of the regulator. The one or more capacitors coupled to the output stabilize the regulator and allow the regulator to operate for more types of load circuits with wider ranges of characteristics.